Mitochondrial replacement therapy could help prevent genetic diseases

Last week, a panel of researchers recommended that the United States government should allow scientists to test a controversial gene therapy technique that requires combining genetic material from three “parents” to make an embryo. Scientists from the U.S. National Academies of Sciences, Engineering, and Medicine released a report that says testing mitochondrial replacement therapy is ethical, but the procedure should only be used to make male embryos.

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Mitochondrial diseases are rare, only affecting a few thousand people a year in the U.S. They can cause a variety of serious health issues, such as seizures, heart disease, developmental delays and dementia. Because mitochondrial diseases are only inherited from the mother, many women who have it often choose to adopt children instead of risking passing the condition down. However, scientists believe that mitochondrial replacement therapy could allow women to have healthy children who are genetically related to them by making sure the embryo has healthy mitochondria donated by a third “parent,” Arielle Duhaime-Ross reports for The Verge.

Every human cell contains mitochondria, organelles which help power the cell by breaking down molecules into energy, kind of like a stomach breaking down food. Mitochondria are also the only part of the cell that contains DNA aside from the nucleus, which is why mutated mitochondria can pose a problem for some people.

Mitochondrial replacement therapy works by taking a healthy nucleus from the egg of a woman with this condition and implanting it into an egg donated by another woman with healthy mitochondria. The egg is then fertilized with the father’s sperm, resulting in an embryo that has three genetic parents, Annalee Newitz reports for Ars Technica.

While critics of the procedure say it could be dangerous to make alterations of an embryo’s genes, mitochondrial replacement therapy does not actually edit the embryo’s genome. The procedure is closer to receiving a donated organ than gene editing, which requires using tools like CRISPR-Cas9 to replace specific genes in a subject’s DNA, Newitz writes. In fact, because mitochondria only contain a tiny amount of DNA, some researchers hesitate at saying embryos that undergo mitochondrial replacement have three parents.

When using mitochondria from a donor egg, the resulting egg (which has the nucleus from the intending mother) has 99.9 percent of its coding DNA from the intending mother. The donor provides the energy necessary for the egg to function normally. When the egg is then fertilized by a sperm cell, the resulting embryo carries less than 0.1 percent of its DNA from the mitochondrial donor.

At the same time, critics argue that the changes are so slight and mitochondrial diseases affect so few people that it isn’t worth risking opening doors to further human genetic editing in the future or might accidentally introduce new genetic errors into the gene pool. That’s why the new report recommends only testing the procedure on male embryos, Rob Stein reports for NPR. However, last year the United Kingdom gave doctors the go-ahead for testing this treatment in clinical trials with no restrictions on the embryo's gender.

The report is a step forward for American scientists interested in pursuing clinical trials for mitochondrial replacement therapy, but the FDA is holding off for now. While the agency praised the panel’s work on the report and is reviewing their findings, it stated that the most recent federal budget "prevents the FDA from using funds to review applications in which a human embryo is intentionally created or modified to include" changes that could be inherited by future generations, Stein reports.

While researchers may be cautiously optimistic about the future of mitochondrial replacement, it may be years before the U.S. government approves any clinical trials, much less treatments.

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About Danny Lewis

Danny Lewis is a multimedia journalist working in print, radio, and illustration. He focuses on stories with a health/science bent and has reported some of his favorite pieces from the prow of a canoe. Danny is based in Brooklyn, NY.